I don't think it's a question of how much of the energy is converted to heat (which is clearly going to be near-100%), but rather what you're heating. If you'r

I don't think it's a question of how much of the energy is converted to heat (which is clearly going to be near-100%), but rather what you're heating. If you're dumping energy into the room until you reach some temperature obviously it doesn't matter much what you're heating or how, but if your goal is merely to have the room feel comfortable to humans, heating up the whole volume to do so is not particularly efficient. That said, I doubt that matters much for the average office space. A far more important factor - especially for those in cold climates - is that the "regular heater" is almost certainly not powered by the electric grid, but rather local combustion. I heat my home with gas, not electricity.